gymbox, London Membership from £78 pm or £20 per visit
Dubbed the hardest class in the world, Flatline tests strength and stamina over 45 minutes. From box jump burpees to rope climbs, you’re faced with five rounds of six gruelling, high intensity moves. the Verdict It’s hard for sure, but it’s not impossibly difficult. There’s no getting off lightly, though – the instructor wore a top with the line ‘I don’t care, work harder’.